Particulate matter filters are increasingly used in automotive emissions systems for reducing particulate concentrations in engine exhaust. However, over time, such filters can suffer irreversible decreases in trapping efficiencies as the filter develops cracks due to uncontrolled temperature excursion during the filter regeneration process, by means of which the soot accumulated in the filter is burned off under controlled engine operating conditions. Losses in trapping efficiency may result in increased particulate matter emissions well above the regulated limit.
Increasingly stringent particulate matter emissions standards and proposed government-mandated on-board diagnostic (OBD) requirements for monitoring the trapping efficiency of a particulate filter have stimulated much research into new techniques for monitoring particulate filter performance. Currently, only laboratory grade instruments are available for particulate matter measurements. Such instruments typically measure particulate concentrations via optical, gravimetric or electrical methods. These instruments typically require controlled operating conditions and extensive calibration for proper functioning. Furthermore, some of these instruments, such as instruments that utilize optical measurement techniques, may require periodic cleaning. Therefore, these instruments may be too expensive and difficult to use under normal automobile operating conditions to be a practical solution to monitoring particulate emissions in automobiles. Furthermore, other methods for detecting a particulate filter failure, such as the differential pressure method in which a pressure differential across the filter is monitored, may not be suitable for detecting a failure of the filter due to interference effects from ash loading in the filter.
The inventors herein have realized that the performance of a particulate filter may be efficiently and effectively monitored by performing, in an apparatus having an internal combustion engine, an exhaust system for transporting engine exhaust from the engine, a particulate filter disposed in the exhaust system, and a particulate detector disposed downstream of the particulate filter in the exhaust system, a method for monitoring a performance characteristic of the particulate filter, including detecting an increase in exhaust flow in the exhaust system; determining if the increase in the exhaust flow occurs at a rate equal to or greater than a threshold rate of change; and if the increase in the exhaust flow is determined to have occurred at a rate equal to or greater than the threshold rate of change, then determining from a signal received from the particulate detector if a value of a parameter related to particulate matter passing through the particulate filter has a predetermined relationship to a predetermined threshold value that indicates a reduction in filter trapping efficiency. In some embodiments, the particulate sensor may include an image charge sensor.